thank you for your help :)

> restart; BB:=piecewise(c>0.75 and c<0.76,cc, c>0.76 and c<0.80, dd, c>2,ee);
> lambda := 3*10^(-12);
> L := 4*10^(-9);
> M := L+(1/100)*L;
> delta := 2*10^(-9);
> d0 := lambda;
> q := .1*Pi;
> AA := M*tan(c);
> da := (d0^2+delta^2)^(1/2)+(M^2+(M*abs(tan(c)))^2)^(1/2);
>
> db = (d0^2+delta^2)^(1/2)+(M^2+(2*delta-M*abs(tan(c)))^2)^(1/2);
>
> h := 1/da^2+1/db^2+2*cos(2*Pi*(da-db)/lambda)/(da*db);
> aa := 1/da^2+1/db^2+(2*cos(2*Pi*(da-db)/lambda)-q)/(da*db);
> bb := 1/da^2+1/db^2+2*cos(2*Pi*(da-db)/lambda)/(da*db);
> cc := 1/da^2+1/db^2+2*cos(2*Pi*(da-db)/lambda)/(da*db);
> LL := subs(db = (d0^2+delta^2)^(1/2)+(M^2+(M*abs(tan(c))-2*delta)^2)^(1/2));
> dd := 1/da^2+1/db^2+2*cos(2*Pi*(da-db)/lambda)/(da*db);
> ee := 1/da^2+1/db^2+(2*cos(2*Pi*(da-db)/lambda)-q)/(da*db);
> ff := 1/da^2+1/db^2+2*cos(2*Pi*(da-db)/lambda)/(da*db);
> plot(BB, c = .51 .. .52);

also, there's no graph appearing in here. but when I do the first two (c>0.75 and c<0.76,cc, c>0.76 and c<0.80,dd)

Here is a successful graph:

> BB:=piecewise(c>-1.57 and c<-0.06, h, .76, h, c>.76 and c<.80,a);
> lambda := 3*10^(-12);
> L := 4*10^(-9);
> M := L+(1/100)*L;
> delta := 2*10^(-9);
> d0 := lambda;
> q := .1*Pi;
> AA := M*tan(c);
> da := (d0^2+delta^2)^(1/2)+(M^2+(M*tan(c))^2)^(1/2);
>
> db := (d0^2+delta^2)^(1/2)+(M^2+(2*delta+M*tan(c))^2)^(1/2);
> h := 1/da^2+1/db^2+2*cos(2*Pi*(da-db)/lambda)/(da*db);
> L := subs(db = (d0^2+delta^2)^(1/2)+(M^2+(2*delta-M*tan(c))^2)^(1/2));
> a := 1/da^2+1/db^2+2*cos(2*Pi*(da-db)/lambda)/(da*db);
> plot(BB, c = .756 .. .82);

BB:=piecewise(c>-1.57 and c<-0.06, h, c>-0.06 and c<-0.05, b);

 Error, (in rtable/Sum) invalid arguments

when I do thisL, it does not give an error

BB := piecewise(c > .75, c < .76, cc, c > .76, c < .80, dd, c > 2, ee)

what's wrong with two negative values?